Glass article and method of making it



Dec. 20, 1966 e. SAWCHUK ETAL 3,293,052

GLASS ARTICLE AND METHOD OF MAKING IT Filed March 14. 1963 0 o O o 0 Q 6 5 4 TIME IN HOURS INVENTORS Y E Kw 0 f HT n s AD V! S M ..Y 2 GE m mm H R a OT L8H United States Patent 3,293,052 GLASS ARTICLE AND METHOD OF MAKING IT Loris G. Sawchuk and Stanley D. Stookey, Corning, N.Y., assignors to Corning Glass Works, Corning, N.Y., a corporation of New York Filed Mar. 14, 1963, Ser. No. 265,150 6 Claims. (Cl. 106-54) This invention relates to the manufacture of glass articles possessing phototropic properties. More particularly, this invention is concerned with the production of glass articles exhibiting phototropicity, i.e., their optical transmittance varies reversible with the intensity of the actinic radiation thereon, where the development of such phototropicity is the result of the presence of molybdenum and/or tungsten in the glass composition.

A rather complete discussion of the theoretical considerations and practical problems involved in the manutacture of phototropic glass articles is presented in the co-pending United States patent application, Serial No. 213,634, filed July 31, 1962, by W. H. Armistead and S. D. Stookey, one of the applicants of the present patent application. Briefly recapitulating the explanation therein, a phototropic glass possesses the inherent characteristic that its optical transmittance varies inversely with the intensity of actinic radiation thereon, this actinic radiation :generally comprising exposure to the ultraviolet and visible light components of natural sunlight. These glasses are distinguishable from the photosensitive glasses known to commerce, i.e., glasses which can be darkened by exposure to ultraviolet radiation succeeded by a heat treatment thereof, in the reversibility of their optical transmittance as they are alternately exposed to and then removed from actinic radiation. The reason for this effect is not completely understood and the explanation postulated by Armistea-d and Stookey, viz., that there is some kind of reaction occurring between the actinic radiation and the suomioroscopic crystals which have been caused to be dispersed in the glassy matrix that alters the absorptive characteristics of the crystals upon visible radiation, is adopted herein. The reversibility of optical transmittance is ascribed to the fact that since these radiation-sensitive crystals are dispersed in a glassy matrix the removal of the activating radiation perm-its the crystals to return to their original state, and, since this matrix is impermeable to and non-reactive with the products formed during such exposure, they cannot diffuse away. Thus, in the usual case, these crystals become darker in hue upon exposure to actinic radiation but re- :gain their original color when the activating radiation is excluded.

Glass possessing phototropic properties has been recommended for use as window panes, automobile windshields, ophthalmic lenses, structural wall panels, and other like applications.

Armistead and Stookey disclosed that silicate glasses of the system R2OB O3'A12O 'SlO where R 0 represents the alkali metal oxides, viz., Li O, Na O, K 0, Rb O,

3,293,052 Patented Dec. 20, 1966 and Cs O, could be made phototropic through the inclusion of silver and at least one halogen of the group chlorine, bromine, and iodine. The base glasses consist essentially, by weight, of about 76% SiO 4'26% A1 0 1-26% B 0 and at least one alkali metal oxide in the indicated proportion selected from the group consisting of 2-8% Li O, 415% Na O, 620% K 0, 825% R'b O, and 10-30% Cs O. Very minor amounts of lowtemperature reducing agents such as tin oxide, iron oxide, copper oxide, arsenic oxide, and antimony oxide may be added to improve the phototropic properties of the glass. Armistead and Stookey also disclosed the possible additions of fluorine, phosphorus pentoxide, and certain bivalent metal oxides such as MgO, CaO, BaO, SrO, ZnO, and PbO. Fluorine is firequently added to the glass batch as a melting aid but its effect on phototropicity of the glass has not been fully resolved as yet. These glasses, as disclosed by Armistead and Stookey, are extremely sensitive to the action of ultraviolet and visible radiation and exhibit exceptional reversibility of optical transmittance when exposed to and removed from such radiation. However, this extreme sensitivity of silver halidecontaining glasses to actinic radiation is a drawback in some applications. Thus, the fact that radiation of very low intensity will cause substantial darkening of the glass may be a decided disadvantage can be readily appreciated when it is understood that in many of these glasses the effect of sunlight in the fairly early morning, i.e., around 8:00 a.m., will be substantially the same as caused by sunlight received at high noon. Thus, it can instantly be observed that there is lacking a proportionality of darkening tendency to the intensity of incident radiation. Such proportionality would be extremely desirable in window panes, wall panels, spectacle lenses, and the like where the variability of transmittance of the glass would yield a more uniform distribution of light intensity to a room interior or to the eye. Hence,

research has been directed toward the discovery of an activating agent which would would yield a phototropic glass wherein there is good proportionality between the intensity of the incident radiation and the degree of darkening in the glass caused thereby.

Therefore, the principal object of this invention is to provide glass compositions exhibiting phototropic properties wherein the variability of the optical density of the glass is closely dependent upon the intensity of the incident actinic radiation thereon.

Other objects will be apparent from the description of the invention set out hereinbelow and from the accompanying drawing which sets :iorth a time-temperature graph of a heat treating schedule usefiul in the invention.

We have discovered that the principal object of this invention can be attained in an article comprising an inorganic silicate glass wherein the inorganic crystals which become darker in color upon exposure to actinic radiation of wave lengths of 3000-5500 A. are composed of silver molybdate, silver tun gstate, and mixtures and/or solid solutions thereof. The behavior of these crystals in inducing phototropicity in silicate glasses is similar to that discussed in detail by Anmistead and Stookey. 'I he minimiun concentration of crystals necessary to produce discernible phototropicity, about 0.005% by volume, is in agreement therewith. Likewise, where the concentration of crystals exceeds about 0.1% by volume and/or the size of the crystals exceeds about 0.1 micron in diameter, a translucent or opalized, rather than a transparent article, is produced.

We have learned that in order to produce the minimum amount of required crystallinity Within the ultimate glass structure, silver must be present in an amount of 0.2% by weight as chemically analyzed and the \glass must also contain, on an analyzed basis, at least 2.5% total of the oxide of at least one heavy metal of the group consisting of molybdenum and tungsten.

We have further discovered that certain glasses within the system Na O-Al O -B O -SiO are particularly suitable in exhibiting phototropicity 'by means of. the inclusion of silver and at least one heavy metal selected from the group consisting of molybdenum and tungsten. Such glasses become darker in color upon exposure to actinic radiation of wave lengths between about 3000-5000 A. (Angstrom Units), i.e., the crystallization developed upon said exposure is sensitive to radiation in the ultraviolet segment of the spectrum and up into about the middle of the visible range. X-ray diffraction analysis of the precipitated crystallization has indicated such to be silver molybdate or silver tungstate. Where mixtures of the metals molybdenum and tungsten are utilized to induce the phototropic behavior, the presence of a silver molybdate-tungstate solid solution is believed to be indicated.

Thus, we have found that a particularly desirable phototropic glass can be produced in a base glass composition consisting essentially, by weight, of about 8-15 Na O, 711% Al O l-28% B 0 and 44-62% SiO the sum of these recited base glass constituents and the components of the silver molybdate and/ or tungstate crystals comprising at least about 90% of the total glass composition.

We have further discovered that the addition of very minor amounts of low-temperature reducing agents, generally in amounts less than 1% by weight, may be beneficial in improving the phototropicity of these glasses. Such agents are: tin oxide, computed as SnO; copper oxide, computed as CuO; iron oxide, computed as FeO; arseni oxide, computed as As O and antimony oxide,

computed as $3 0 Fluorine and P 0 may be added to the glass batch to improve its melting qualities and to inhibit devitrification upon cooling. The effect of fluorine upon the phototropicity of the glass is not completely known but the amount utilized is kept low in order to forestall the precipitation of fluorides within the glass. The P 0 content is also held low so that its action as an oxidizing agent will be minimized.

The inclusion of the bivalent metal oxides should be limited to not more than about 4% MgO, 6% C210, 7% SrO, 8% BaO, 8% 2110, and 8% PbO, on a weight basis, the total amount of these bivalent metal oxides not exceeding 10%, by weight, of the glass composition.

The constituents of the base glass, i.e., the Na O, A1 0 B 0 and SiO are preferably maintained within the ranges set forth hereinabove to assure the production of a glass possessing good phototropic and other physical properties. Where the silica content is less than about 44%, there is sometimes a tendency for undesirable crystalline phases to precipitate along with the molybdates and tungstates. In batches where the silica content is in excess of about 62% or the minimum amount of Na O is absent, the glass is very difiicult to melt satisfactorily at conventional melting temperatures. Where the amount of B 0 exceeds about 28% or the Na O content is greater than about the glass is subject to chemical attack or Weathering. At least 10% B 0 must be present in the glass to secure the precipitation of the desired silver molybdate and/ or tungstate crystals. A1 0 must be present within the aforementioned range in order to insure the absence of undesirable glassy or crystalline phases which must form with, or in preference to, the silver molybdates and tungstate's.

The production of the phototr'opic articles of this invention contemplates melting together the components of the desired crystalline phase with the components of the base glass and thereafter precipitating said crystals in situ in a glassy matrix. Thus, glasses of the desired composition may be obtained in accordance with conventional glass practice by melting the required batch in a crucible, pot, or tank. Thereafter, the melt is cooled and a glass shape of the desired configuration formed therefrom employing any of the conventional glass-forming techniques such as blowing, casting, drawing, pressing, rolling, etc., and then cooled to room temperature, this cooling step frequently being supplemented with an annealing step. The radianon-sensitive crystals can be precipitated upon cooling the melting to a glass body. However, the crystals produced thereby are often nonhomogeneous and of nonuniform size. Therefore, the preferred method comprises cooling the glass to at least below its annealing point (475 "525 C.) at so rapid a rate that no crystallites of the proper size, or at least an insufiicient number thereof, are precipitated to cause a discernible phototropic effect in the glass. Following this quick cooling step, the glass shape is subjected to a special heat treatment by means of which the precipitation of submicr-oscopic crystals can be closely controlled both as to homogeneity and size, The most effective crystal size appears to range from about 40-200 A. This heat treatment consists of exposing the glass shape to a temperature above the strain point of the glass (425475 C.) for a time sufiicient to attain the desired crystallization therein such that the shape will demonstrate satisfactory phototropicity. This heat treatment normally comprises heating the glass shape to a temperature of about 500-900 C. for a period of time ranging from as little as about /2 hour at 900 C. to as long as 24 hours at 500 C. It is believed that the heat treatment permits the rearrangement of the silver cations and the molybdate and tungstate anions thereby developing a separate crystalline phase of the desired silver salt within the glass matrix. This rearrangement will occur more readily at higher temperatures, principally because the viscosity of the glassy matrix decreases with an increase in temperature, thereby lessening the resistance to movement necessary in furthering the rearrangement. Hence, a much shorter heating period at the higher temperatures will involve comparable rearrangement as a long period at a lower temperature. Nevertheless, as there are other possible reactions which can occur during the heat treatment such as agglomeration and precipitation of other crystalline phases, the heat treatment at the higher extreme of the operable range must be of limited duration to pre clude such undesirable secondary reactions. After heat treatment, the article is cooled to room temperature, pref erably in a controlled manner such that the glass is annealed.

Table I sets forth examples having compositions falling Within the prescribed ranges as analyzed on the oxide basis in weight percent. The batch constituents may consist of any materials, either oxides or other compounds, which, on being fused together, are transformed to the desired oxide compositions in the necessary proportions.

In accordance with conventional analytical practice, although it has been determined that a substantial portion, if not all, of the silver is present in the glass as ions thereof probably having bonds with oxygen and/or the molybdate and tungstate ions, and not as metalli silver, it is expressed in Table I as silver. The glasses outlined in Table I can be melted from batches in the conventional manner but allowance must be provided for volatilization of silver, this loss being, perhaps, as high as 30% depending upon the melting unit and temperatures employed,

TABLE I 16 17 1s 19 21 22 2a 24 26 27 2s 29 3:? 3:? 3:? 3:? iii 3:? 3:2 3:2 3:? 31% 3:2 14.5 13.0 12.0 19.0 19.0 19.0 19.4 19.4 18.4 19.4 19.4 12.7 14.5 13.0 9.1 9.1 9.1 9.6 9.6 2.; 9.6 9.6 312 "5.2- 0.30 0.66 0.60 0.66 0. 54 0. 45 0. 45 0. 36 0. 54 0.36 0.27

The glasses recorded in Table I were obtained by com- Table II pounding conventional batch ingredient in suitable pro- 25 Example N0: Heat treatment portions to produce the desired glass composition (making 1 for 2 hours allowance for the volatilization of silver), ball milling 2 for 2 hours these ingredients to insure a homogeneous melt, and then 3 C. for 2 hours melting the batch at a temperature of 1350l400 C. for 4 I C. for 2 hours about 6 hours. The melts were then poured and rolled 5 I for 2 hours into plates, these plates being annealed at about 500 C. 6 650 C. for 16 hours In each instance, the plates were cooled to room tempera- 7 h C for 16 hours ture for visual inspection and testing for phototropicity. 8 c for 2 hours In all cases, the rapidity of cooling occasioned by the 9 for 2 hours rolling into plates was adequate to preclude the develop- 1O for 2 hours ment of suflicient crystallization to cause any appreciable 11 750 for 2 hours phototropicity. The plates were then heat treated to pro- 12 for 2 hours mote a controlled growth of radiation-sensitive crystals. 13 750 for 2 hours It will be appreciated, however, that the glass article need 14 750 for 2 hours not be cooled to room temperature prior to heat treatment 15 for 2 hours but may merely be cooled to the transformation point of 16 750 for 2 hours the glass, i.e., that temperature at which the melt is 17 750 for 2 hours deemed to have become an amorphous solid, and then 18 750 for 2 hours subjected to heat treatment. It will be realized further, of 19 750 for 2 hours course, that in most instances the maximum heat treating 4.5 20 750 for 1 hour temperature which will be utilized with a particular glass 21 750 for 1 hour will not exceed the temperature at which excessive thermal 22 750 for 1 hour deformation occurs. However, it will be understood that 23 750 for 1 hour some fonning methods inherently contemplate a thermal 750 f 1 h deformation of the glass body and here, perha s, the h at 24 a or 1 h treating step could be incorporated therewith, 25 for h Table II sets forth the heat treating tem t mi. 26 for 2 h lized in developing phototropicity in the glasses de ib d 27 750 for 2 Ours in Table I. The heating rate employed in bringing the glass article from room temperature to the heat treating temperature does not appear to have a critical effect on the results. The bodies may be plunged directly into a furnace set at the desired :heat treating temperature, if the size and shape of the piece is such that breakage due to thermal shock does not occur, or they may be heated at substantially any rate. Likewise, the articles may be cooled at substantially any rate as long as they are not damaged through thermal shock or produce undesirable residual stresses. In Examples 1-5 and 12-27, the glass plates were plunged into a furnace, 1he1d thereat for a time suflicient to precipitate submicroscopic crystals of radiation-sensitive material, and then removed from the furnace "and allowed to cool to room temperature in the ambient atmosphere. The remaining glass plates were placed in a furnace, heated at about 5 C./ minute to the desired temperature, held thereat for a time sufficient to precipitate submicroscopic crystals of radiation-sensitive material, and then the heat was cut off and the furnace allowed to cool at its own rate with the plates retained therein.

Table I illustrates the variations in base glass com 5 position which can be made phototropic through the inclusion of silver and the molybdate and/or tungstate ion. The Table also demonstrates the workable "amounts of radiation-sensitive ingredients which are needed in these base glasses. The quantity of activating agents which is operable in this invention is limited by two factors. Obviously, a certain minimum amount must be present to develop sufi'icient crystallization to cause phototropicity. The maximum which can be tolerated is 'based upon the solubility of these agents in the glass. It is apparent from the Table that the amount of molybdate and/or tungstate ions which can be present may be in excess of heat required to combine stoiohiometrically with the silver. However, laboratory testing has shown that at above about 0.8% silver and above about 10% molybdenum or tungsten oxide, excessive precipitation occurs during the quenching step such that the glass becomes inhomogeneous. Some precipitation can be tolerated in the form of a translucent or opalized phototropic glass but excessive precipitation dampens the development of submicroscopic radiation-sensitive crystals simultaneously shaping an article therefrom and cooling to room temperature, the article is placed in a furnace and heated at C./minute to 750 C., held there-at for two hours, the heat to the furnace thereafter cut off and the furnace allowed to cool at its own rate (approximately 2" C./rninute) with the article retained there- What is claimed is:

.1. A phototropic article comprising a body of a silicate glass having in at least a portion thereof crystals of at least one silver compound selected from the group consisting of silver molybdate, silver tungstate, and silver molybdate-tungstate solid solution, said portion of the glass containing, by weight on the analyzed basis, about 0.2-0.8% silver and 25-10% total of the oxide of at least one heavy metal selected from the group consisting of molybdenum and tungsten.

.2. A glass composition which is potentially phototropic consisting essentially, in Weight percent on the analyzed basis, of 8-15% Na O, 711% A1 0 -28% B 0 4462% SiO 0.20.'8% silver, and 25-10% total of the oxide of at least one heavy metal selected from the group consisting of molybdenum and tungsten, the sum of the recited base glass constituents, silver, and heavy 8 metals being a least of the total glass composition.

3. A method of manufacturing a phototropic glass body which comprises the steps of melting a batch for a silicate glass composition, said batch containing 0.2- 08% by weight of silver on an analyzed basis and 2.5- 10% by weight, total of the oxide of at least one heavy metal selected from the group consisting of molybdenum and tungsten, simultaneously cooling the melt and forming a glass article therefrom, subsequently heat treating said glass article at a temperature above the strain point of the glass for a time sufiicient to precipitate submicroscopic crystals of radiation-sensitive material, and then cooling s-aid article to room temperature.

4. The method according to claim 3 wherein the submicroscopic crystals of radiation-sensitive material consist of at least one silver compound selected from the group consisting of silver molybdate, silver tungstate, and silver mol-ybdate-tunstate solid solution.

5. The method according to claim 3 wherein the temperature of heat treatment ranges from about 500 to 900 C.

6. The method according to claim 5 wherein the time sufiicient to precipitate submicroscopic crystals of radianon-sensitive material ranges from about 0.5 hour 'at 900 C. to about 24 hours at 500 C.

References Cited by the Examiner UNITED STATES PATENTS HELEN M, MCCARTHY, Acting Primary Examiner. 

1. A PHOTOTROPIIC ARTICLE COMPRISING A BODY OF A SILICATE GLASS HAVING IN AT LEAST A PORTION THEREOF CRYSTALS OF AT LEAST ONE SILVER COMPOUND SELECTED FROM THE GROUP CONSISTING OF SILVER MOLYBDATE, SILVER TUNGSTATE, AND SILVER MOLYBDATE-TUNGSTATE SOLID SOLUTION, SAIID PORTION OF THE GLASS CONTAINING, BY WEIGHT ON THE ANALYZED BASIS, ABOUT 0.2-0.8% SILVER ABD 2.5-10% TOTAL OF THE OXIDE OF AT LEAST ONE HEAVY METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM AND TUNGSTEN. 